The Dutch diplomat Max Kohnstamm, who has died aged 96, was the last member of an extraordinary group of political visionaries who became known as the founding fathers of the European Union. In the 1950s he played a major part in the launch and development first of the European Coal and Steel Community (ECSC) and then of the European Economic Communities.

Born in Amsterdam, he was the son of Philip Kohnstamm, a professor of theoretical physics and a colleague of Albert Einstein, Paul Ehrenfest and other luminaries of the period who were regular visitors to the family home. Max was educated at Amsterdam University and in the US.

His burning ambition for European integration was born out of traumatic personal experience during the second world war. Max, whose family was part-Jewish, was imprisoned in a concentration camp in the Netherlands whose inmates would be regularly selected for execution in reprisals for Dutch resistance actions. During a period of internment in another camp, he met Kathleen Sillem, a young nurse of mixed Dutch and Irish background. They married clandestinely in 1944. After liberation, Queen Wilhelmina of the Netherlands, having been told that Max was one of the brightest young intellectuals of his generation, asked him to act as her private secretary. When she heard his story, she helped organise a second, formal, wedding ceremony.

It was during this period that Max met Jean Monnet, the charismatic French leader of the movement for European integration. He invited Max to join him on a visit to Germany. The destruction and impoverishment he witnessed convinced Max that Europe must break the cycle of ever more destructive wars. He joined Monnet in campaigning for the Schuman Plan (named after the then French foreign minister, Robert Schuman), which led to the creation of the ECSC. He served as first secretary of the ECSC High Authority between 1952 and 1956.

In 1956 Monnet and Max launched the Action Committee for the United States of Europe, with Max becoming secretary general and then vice-president until 1975. The committee drew on a broad coalition of politicians (Max was a member of the Dutch Labour party), intellectuals, artists and young activists, who saw the creation of the common market as the first step towards "an ever closer union" of the states and people of Europe, in the words of the founding Treaty of Rome in 1957.

Max's fluency in French, English and German – and, later, Italian – helped him establish close personal links with politicians, writers, social reformers, academics and others convinced of the European cause from across the continent. While calmly realistic about Britain's ambivalent role in the European Union, Max remained convinced that in the end, the British would understand that the future lay with Europe.

From 1975 to 1981 he was the principal of the European University Institute in Florence. Max believed that theory should lead to practice and not substitute for it. He was frequently consulted by successive generations of European Union leaders for advice. But his vision of the future never stopped at the borders even of a vastly expanded EU after the end of the cold war. He frequently cited the words of Monnet that European integration would itself only be "a step on the road to a new global order".

A modest and self-effacing man, Max remained an active participant in the fortunes and misfortunes of European integration well into old age. I personally appreciated his clarity and longterm vision when, in his 80s, he joined Stanley Crossick and myself in establishing a new, pro-European thinktank in Brussels, the European Policy Centre, where he became life president. When he retired, his home in the Belgian Ardennes was always open to visitors of all ages and backgrounds.

He is survived by Kathleen, two sons, three daughters and eight grandchildren.